Often in industrial and laboratory practice, there is a need to accurately determine the mass of small dielectric objects of irregular shape. This is difficult to accomplish when objects are too hot, too cold, or too fragile for contact with test equipment. Examples of objects where mass or weight is difficult to measure includes drops of molten polyester or other plastic or polymeric materials; frozen objects such as hydrogen pellets; needle-shaped dielectric objects; and continuous filaments or threads of dielectric materials. A microwave resonant cavity offers a potential solution for these kinds of problems.
A hollow metal cavity is brought into resonance when the wavelength of the coupled electromagnetic wave corresponds with the dimensions of the cavity (Harrington, Time-Harmonic Electromagnetic Fields, p. 321, 1961; Waldron, The theory of Waveguides and Cavities, p. 75, 1967). When a dielectric object is inserted into the cavity, the resonant frequency will shift toward lower frequencies, and the Q-factor of the cavity will decrease. These two effects can be found by sweeping the operating frequency and observing the transmitted energy at the resonant frequency and a number of frequencies around it. Parameters of the cavity depend upon the volume, geometry, and mode of cavity operation, as well as on the permittivity, shape, dimensions, and location of the object inside the cavity. For a given cavity and material sample of regular shape and well-defined dimensions, one can determine the permittivity of the material from equations developed from perturbation theory. This approach has been extensively used for measurement of dielectric properties of materials for many years. Altschuler (Handbook of Microwave Measurements, eds. M. Sucher and J. Fox, p. 530-536, New York: Polytechnic Press, 1963) and Bussey (Proc. IEEE 55(6), 1046-1053, 1967) discussed the use of microwave resonant cavity techniques to measure the microwave dielectric properties of materials by measuring the shift in the resonant frequency and the change in the Q-factor of the cavity.
Microwave resonant cavities have also been used for evaluating the dielectric properties of geometrically defined samples when the cavity is calibrated with dimensionally identical samples of various known permittivities (Rueggeburg, IEEE Trans. Microwave Theory Techn., MTT-19, 517-521, 1971). By measuring a fiber in two resonant cavities, the dielectric constant and diameter of the fiber may be determined, or its moisture content may be determined independent of its diameter or density (Lakshminarayana et al., IEEE Trans. Microwave Theory Techn., MTT-27, 661-665, 1979; Hoppe et al., IEEE Trans. Microwave Theory Techn., MTT-28, 1449-1452, 1980).
Talanker and Greenwald (Rev. Sci. Instru. Vol. 59(7), p. 1085-1087, July, 1988) disclose a method for determining the mass of frozen hydrogen pellets using a resonant microwave cavity. This method uses the resonant cavity to control the frequency of an oscillator. The output of the oscillator is mixed with the output of a second, local oscillator to provide a one-parameter measurement which provides information related to the volume of the hydrogen pellet which passes through the cavity because of the frequency shift resulting from the influence of the object on the resonant frequency of the cavity. This method is only useful for objects of similar shape and dielectric constants. Furthermore, this method requires knowledge of the density of the object in order to determine the mass of the object from the volumetric information provided by the measurement.
A resonant cavity has been applied for determining moisture content in uniformly shaped single seeds by simultaneous measurements of resonant frequency shift and the transmission factor (Kraszewski et al., IEEE Trans. Instrum. Meas., Vol. 38(1), 79-84, 1989; J. Agric. Engin. Res., Vol. 48, 77-87, 1991; U.S. Pat. No. 5,039,947 ('947), 1991). Kraszewski et al., 1989, disclose a nondestructive process for the determination of moisture content in single soybeans using a microwave resonator. A seed is placed in a microwave resonant cavity and the resonant frequency shift and change in Q-factor are measured. This process allows the measurement of moisture content of articles of nearly uniform spherical shape. Kraszewski et al., 1991 and '947 disclose a nondestructive process for determining the moisture content of articles of irregular or variable shape where the irregular or variable-shaped product is inserted into a microwave resonant cavity in a first position and the energy dissipated in the product and the shift or change in the resonant frequency (or wavelength) due to the presence of the product is measured. The orientation of the product is then changed to a second position which is rotated by about n.times.90 degrees with respect to the maximum field vector (n is an odd integer) and the measurements are repeated.
Kraszewski et al. (American Society of Agricultural Engineers, Paper No. 92-6505, 1992; Trans. ASAE, Vol. 36(1), 127-134, 1993) disclose a method for the simultaneous measurement of moisture content and mass in single peanut kernels, which are also of nearly uniform shape, using microwave resonator measurements of resonant frequency and change in cavity transmission characteristics. The cavity consisted of a section of standard WR-284 rectangular waveguide (inside dimensions: 72.times.34 mm) 305 mm long operating in the H.sub.105 (TE.sub.105) mode. It was coupled with external waveguides through two identical coupling holes 20.6 mm in diameter at each end of the cavity. A PLEXIGLAS.TM. tube of 15.8 mm outside diameter and 12.4 mm inside diameter was installed in the center of the cavity which supports the peanut kernel at the center of the cavity.
While various methods have been developed for measuring microwave properties of different materials including the mass of uniformly shaped objects, there remains a need in the art for a method for rapid determination of arbitrarily shaped objects independent of their size, density, and dielectric constant, especially objects which can not be handled. The present invention provides a method which is different from prior art methods and solves some of the problems of mass determination of articles, especially those which can not be handled.